Cassiopea xamachana

ORDER Rhizostomeae




Cassiopea xamachana R. P. Bigelow, 1892, Jamaica, West Indies.


None known.


Medusae in the genus Cassiopea are unique in resting bell side down on the ocean bottom. The swimming bell may reach 6 in (15 cm) in diameter. It is flattened, with 40 lappets. The oral arms are about 1.5 times the bell radius. They branch laterally and have numerous tiny tentacle-like projections. The upside-down medusae appear to be clumps of algae because of the bushy greenish brown oral arms that cover the bell. The topside of the bell can be marked boldly with regular spots and stripes in brown and blue. Tentacles are lacking.


This species is found in the eastern tropical Atlantic, including Florida, the Caribbean, and Mexico. Other members of the genus occur in tropical waters worldwide.


These jellyfish are found year-round in sunny, shallow tropical lagoons and mangroves and in coral back-reef areas.


Although the medusae rest bell side down on the bottom, regular swimming pulsations are important in terms of feeding and water exchange for respiration. The medusae do not swim in the water column unless they are disturbed.


Nutrition of the upside-down jellyfish depends on a combination of zooplankton feeding and symbiosis with intracellular al gae (zooxanthellae). Pulsations of the bell sweep small epiben-thic crustaceans and zooplankton into the oral arms, where they are caught by the numerous tiny tentacle-like projections and passed to the many small mouths along the oral arms. The greenish brown color of the oral arms comes from the zooxan-thellae in the tissues. These algae contribute photosynthetic products to the medusa's nutrition. The symbiosis represents a mutual exchange of nutrients that, together with zooplankton, supports algae and medusa metabolism, growth, and reproduction. The polyps also contain zooxanthellae and feed on small crustaceans.


The life cycle is typical of rhizostome scyphozoans. Metamorphosis of the larvae into polyps requires a peptide derived from the cell walls of decomposing plants, such as mangroves. Polyps can reproduce asexually by budding off chains of special individuals that settle to form polyps. Strobilation produces only one ephyra per polyp.


Not listed by the IUCN.


The creation of protected lagoons for resorts or aquaculture and associated eutrophication (nutrient pollution) of those areas have resulted in increased medusa populations in the Florida Keys and Mexico. The medusae are harmless to humans but can be irritating if they are handled. ♦

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